1. Field of the Invention
This invention relates to a zoom lens, and particularly to a zoom lens which comprises a magnification changing system including three groups or portions, i.e., a focusing portion having a positive refractive power, a variator portion having a negative refractive power, and a compensator portion having a negative refractive power, and a relay lens system.
2. Description of the Prior Art
Recently, by virtue of the progress of image pickup tubes, video cameras have been obtained which are sufficiently good in resolution, color balance and various image pickup characteristics for conventional 1-inch tubes and 2-inch tubes and even for 2/3-inch tubes. For use with such tubes, a need has arisen for compact and light-weight zoom lenses having a large aperture and a high zoom ratio. Regrettably, however, the zoom lenses commercially available have not been sufficiently compact and light in weight and have not had a sufficiently large aperture, and zoom lenses having higher performance have become necessary.
Generally, in the designing of zoom lenses, it is desired to reduce the total length of the lens and minimize the number of lenses in order to achieve compactness and light weight, but various difficulties have been encountered in realizing these aims.
First, in order to reduce the total length of the lens, it is indispensable to make the magnification changing system small, but if an attempt is made to make the magnification changing system small by intensifying the refractive power of the variator which, among the groups forming the magnification changing system, is regarded as the most effective to make the magnification changing system small, a sharp curvature would be provided to each surface of the lenses forming the variator and this would result in an increased amount of aberrations. Under such conditions, it has been very difficult to provide adequate correction of spherical aberration, coma and astigmatism over the entire magnification change range.
Also, in the relay lens system, when used as the optical system for a video camera or the like, it is necessary to place the exit pupil of the lens system substantially at an infinity position, that is, to make the system into a telecentric optical system and thus, the power arrangement of the relay lens system is limited and, necessarily, the compactness of the zoom lens is limited.
It has also been very difficult to realize a large aperture by using a limited number of lenses.